An engine having a turbocharger generally utilizes the energy of exhaust the gas from the engine to rotate a turbine which rotates a compressor connected in a coaxial relationship with the turbine to increase the amount of air introduced into the cylinders of the engine, thereby supplying supercharged air having positive pressure at atmospheric pressure or more to the engine to improve the torque thereof.
In such an engine having the turbocharger, since the pressure of the supercharged air can not be increased excessively in consideration of durability of the engine, a bypass mechanism is provided which bypasses the turbine to decrease the amount of exhaust gas supplied to the turbine from the engine.
On the other hand, the pressure of the supercharged air is determined by the capacity of the turbine. Accordingly, when a small turbine is used, torque of the engine at a low speed is improved but torque at a high speed is reduced, whereas when a large turbine is used, torque of the engine at a high speed is improved but torque at low and middle speed is reduced. Thus, there has been proposed a variable geometry type turbocharger, for example, in Japanese Utility Model Application Laid-open No. 50310/78, in which the amount of exhaust gas introduced into the turbine can be changed in accordance with operating conditions of the engine to increase torque of the engine in a range from a low speed operation to a high speed operation of the engine.
Such a variable geometry type turbocharger changes a sectional area of a throttle throat of a scroll forming a turbine by means of a flap valve disposed in the throttle throat to vary flow velocity of exhaust gases introduced into the turbine, thereby maintaining the supercharged air at an outlet of the compressor to a predetermined pressure and increasing torque of the engine in a range from a low speed operation to a high speed operation.
However, with such a prior art variable geometry type turbocharger, when the sectional area of the throttle throat of the turbine is reduced by closing the flap valve in order to increase pressure of the supercharged air at a low engine speed, pressure of the exhaust gas at the throttle throat of the turbine is increased at an early stage, so that the bypass valve of the bypass mechanism is open before a predetermined amount supercharged air is obtained, thereby degrading the supercharged air and reducing torque of the engine.